A variety of dimmers for fluorescent lighting have been proposed. Examples can be found in U.S. Pat. Nos. U.S. Pat. No. 3,264,518 to Stauverman, U.S. Pat. No. 3,614,527 to Wirtz, U.S. Pat. No. 3,819,982 to Nelson, U.S. Pat. No. 3,935,505 to Spiteri, U.S. Pat. No. 4,096,413 to Alley, U.S. Pat. No. 4,172,981 to Smith, U.S. Pat. No. 4,207,498 to Spira et al, U.S. Pat. No. 4,277,728 to Stevens, U.S. Pat. No. 4,894,587 to Jungreis et al, U.S. Pat. No. 4,928,038 to Nerone, U.S. Pat. No. 5,175,477 to Grissom, U.S. Pat. No. 5,194,781 to Konopka and U.S. Pat. No. 5,208,513 to Murayama.
Although various dimmers have been proposed, prior art dimmers fall into two principal categories: electronic ballasts adapted to apply a continuous AC voltage of variable amplitude to a fluorescent lamp; and dimmers that apply phase control to the AC line voltage used to power a fixture and operate with magnetic ballasts. Electronic ballasts can operate fluorescent lamps at very low power levels for extended periods of time with no apparent damage to the lamps. However, retrofitting existing fluorescent fixtures can be very costly.
Dimmers that apply phase control can be conveniently connected to fluorescent light fixtures operating with a magnetic ballast. Such dimmers typically involve a controllable switch (often a triac or paired silicon controlled rectifiers) in series with the fixture, a potentiometer or other device permitting a user to specify a desired phase angle (power setting), and a triggering circuit that actuates the switch in each half-cycle of the applied AC voltage at the specified phase angle. The terms "phase control" and "applying phase control to an AC voltage" as used in this specification should be understood as controlling conduction in response to an AC voltage so that conduction occurs in each half-cycle of the AC voltage only after a particular phase angle. Terms indicating application of more or less phase control to an AC voltage should be understood as increasing or decreasing the phase angle used for phase control respectively to decrease or increase power consumption. The term "no phase control" should be understood as applying the AC voltage with a 0 phase angle.
There are several problems associated with using phase control to vary power consumption of a conventional fluorescent fixture. A fluorescent lamp is inherently a pulsating device but persistence in human vision gives the appearance of constant illumination. In a range of power settings between 70% and 90% of maximum power, however, a fixture is subject to flickering. The terms "flicker" and "flickering" as used in this specification should be understood as pulsing of light, perceptible to the human eye. The inventors' experimentation has indicated that flickering is most likely to occur between 80% and 85% power settings (lower phase angles). In a range of power settings that typically occurs below a 50% setting (larger phase angles), the fluorescent lamp turns gray and becomes progressively darker with increasing phase control. The inventors have identified that continued operation in a gray state results in a very rapid bum-out of the lamp, and that a lamp can fail in as little as 48 hours depending on exact power settings. These effects depends very much on the nature of the ballast, the nature of the lamps, and aging of the components. The range of power settings subject to flickering appears to expand with age, and the power setting at which a gray state occurs appears to increase with age. Ambient temperatures appears also to affect such conditions.